Ice flow on deforming sediments: Ice stream B- and Lake Michigan?
wires at the same site. The different trends during this period are partially due to transient effects, but these effects tend to diminish with time. The difference may be primarily due to crevassing or other inhomogeneity in the near-surface strain field. While the wires at this site showed different trends in the strain rates, two wires at a different site gave the same strain rates. The most striking feature of figure 2 is the diurnal oscillation in the strain rates. We have sufficient control to eliminate virtually all possible instrumental effects. The diurnal variation is intermittent and does not occur at all sites. The seismic event rate also showed intermittent diurnal behavior, as seen in figure 3. Moreover, there was noticeably more seismic activity in winter than in summer. The locations of the seismic events were not determined and could have been near the surface. The diurnal behavior, at least in the strain rate, is a surprising and as yet unexplained feature. We are doubtful that it can be explained by thermal stresses, as determined by Sanderson (1978).
Ice flow on deforming sediments: Ice stream Band Lake Michigan? RICHARD B. ALLEY
Earth System Science Center
and
Department of Geosciences Pennsylvania State University University Park, Pennsylvania 76802
The existence of a deforming bed beneath ice stream B (Alley et al. 1986) remains probable but unproven. Identification of the basal mechanism of ice-stream flow will allow us to concentrate on finding its parameters for numerical models, a development which will lead to improved predictions of the rate at which the ice sheet will respond to perturbations such as those possibly caused by greenhouse warming or those already triggered by past climatic changes. Ongoing field studies associated with the Siple Coast Project seek to identify the mechanism of ice-stream lubrication unequivocally (Engelhardt et al. 1990). Over the last year, I refined a possible one-dimensional steady model for basal behavior and found that this model is consistent with available data (Alley 1990); this consistency continues to favor the deforming-bed hypothesis. This model suggests that ice streaming is initiated when enough basal meltwater accumulates to raise basal water pressures quite close to flotation; a poorly consolidated bed that erodes easily to produce 1990 REVIEW
This research was supported by National Science Foundation grant DPP 87-16604.
References
Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1986. Seismic measurements reveal a saturated porous layer beneath an active Antarctic Ice Stream. Nature, 322, 54-57. Engelhardt, H.N. Humphrey, B. Kamb, and M. Fahnstock. 1990. Physical conditions at the base of a fast moving Antarctic Ice Stream. Science, 248, 57-59.
Harrison, W.D., and K.A. Echelmeyer. 1989. Short-term variations in the speed of ice stream B, Antarctica. Antarctic Journal of the U.S., 24(5), 81-82.
McDonald J., and I Whillans. In press. Search for short-term velocity changes of ice stream B, West Antarctica. Journal of Glaciology. Sanderson, T.J.O. 1978. Thermal stresses near the surface of a glacier. Journal of Glaciology, 20, 257-283.
sediments enhances lubrication and causes ice streaming upglacier of the point where it would begin over hard bedrock. If the geothermal flux is low or subglacial aquifers are efficient, ice streaming can be suppressed even if the bed is melted. There may be two steady states for ice-surface profiles in some regions: • a steep surface slope that causes rapid internal ice deformation but drives basal meltwater through subglacial aquifers so it does not lubricate ice flow, and • a gentle surface slope that slows internal ice deformation but allows meltwater to accumulate at the bed and lubricate basal sliding or bed deformation. The next step in this modeling effort, now in progress, is to conduct time-dependent simulations and check for consistency with available data. If the model still appears to be adequate, then we can begin to assess the stability of the ice-stream system. Of course, further data from the ongoing Siple Coast field programs are the most direct and valuable test of icestream dynamic controls, but modeling provides a useful adjunct. The probability of a deforming bed beneath a large antarctic ice stream raises further interest in the fast-moving ice that drained the Laurentide ice sheet during the Late Wisconsinan, and that deposited widespread till sheets. The antarctic deforming-bed model leads to a better understanding of how the deposits of a deforming-bed glacier might.appear. Comparison of predictions of the deforming-bed model and of other basal models with observed characteristics of basal tills deposited by the Lake Michigan lobe in Illinois shows that the deformingbed model fits the observations better than other models. It is an interesting possibility that ice stream B and the Lake Michigan lobe may be quite similar in ice and sediment dynamics (Alley et al. 1988; Alley in press). This work was supported in part by National Science Foundation grant DPP 87-16016. 79
References Alley, R.B., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1986. Deformation of till beneath ice stream B, Antarctica. Nature, 322, 5759. Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1988. Glacial deposits as (lack of) evidence for deforming beds. (Abstract). SEPM Midyear Meeting Abstracts, 5, 2.
Borehole geophysical observations on ice stream B, Antarctica HERMANN ENGELHARDT, NEIL HUMPHREY, and BARCLAY KAMB Division of Geological and Planetary Sciences California Institute of Technology Pasadena, California 91125
In recent years, the development of the hot-water drilling technique for rapidly drilling deep boreholes through cold ice to the base of the antarctic ice streams has opened the possibility to study the controlling mechanism for fast ice streaming flow. This technique has been successfully tested and implemented during the 1988-1989 austral summer (Engelhardt et al. 1989, 1990; Kamb 1990). In the 1989-1990 austral summer, six boreholes about 1,060 meters deep were drilled on ice stream B (83.5°S 138.2°W). In this article, we report on the geophysical experiments and observations made in these boreholes. Sampling of subglacial material. Samples for studying the physical properties of bottom materials were extracted by four different methods. Two of them, the jet sampling technique and the adhesion method, were already used in the previous field season (Engelhardt et al. 1989). This year, two new devices were added: • The split-tube corer—a short, split piece of heavy tubing, 62 millimeters in diameter—is driven into the bottom sediments using a heavy weight. The split tube can be readily opened for removal of the core material. Samples of subglacial sediment 0.2 meter long were obtained. • The piston corer is 6.5 meters in overall length. The core tube, 50 millimeters in diameter with piston to fit, is 3.5 meters long; the rest of the length is a piece of heavy steel (Shelby) tubing to drive the corer into the bottom. Although the piston corer is more difficult to handle than the other sampling devices, this sampling technique proved to be highly advantageous. Four cores were retrieved: lengths 1.3, 2.0, 2.0, and 3.4 meters. Preliminary examination of the cores show the general character of the material. It is a glacial till—a pebbly, sandy, silty clay with a wide range of grain sizes typical of till, and lacking bedded structure. Clasts up to 5 centimeters in size are present; they are mainly granitic, but metamorphic lithologies are also present. Scarce shell fragments are visible macroscopically, 80
Alley, R.B. 1990. Multiple steady states in ice-water-till systems. Annals of Glaciology, 14, 1-5. Alley, R.B. In press. Deforming-bed origin for southern Laurentide till sheets? Journal of Glaciology. Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving Antarctic ice stream. Science, 248, 57-59.
and microscopic organic remains (sponge spicules, diatoms, etc.) are present though not abundant (Scherer 1989). The till, therefore, is derived, at least in part, from open water marine sediments. A wide variety of source materials has been mixed together, as can be expected in a till. The directly measured porosity of a sample from near the top of the till is 40 percent. This value is high for a till and probably indicates that the till has been dilated by recent shear. A hydraulic conductivity of 2 x iO per millisecond was measured on a reconstituted sample. This low value, typical of tills, precludes hydraulic conduction through the till layer as a significant contribution to water flow at the base of the ice stream. Preliminary mechanical tests have been carried out on till samples from which the coarser rock fragments (greater than 5 millimeters) have been removed. The shear strength is 0.020.04 bar, varying from sample to sample. The till behaves like a perfectly plastic material with this low failure strength. Because of the low hydraulic conductivity, the shear strength measured on the core samples is probably close to the in situ value under the ice stream. The strength of the till is so low that the till should be deforming under the ice, if the basal shear stress approximates the regional average value of 0.2 bar. In fact, the till is by a wide margin too weak to support the average basal shear stress. Therefore, the mechanical support and stability of the ice stream are called into question at the locality studied (Upstream B). The areal coverage of the investigation must be widened to provide a set of representative basal conditions broad enough to account for overall mechanical equilibrium of the ice stream. Till thickness. The length of the longest core extracted from the till shows that the till is at least 3.4 meters thick. The depth of the till was sounded by using the hot-water drill, which penetrates into the till by hydraulic action. The drill went 5 meters into the till without any indication of encountering a consolidated, impenetrable bottom. In two boreholes the drill stem penetrated at least its full length (3.6 meters) into the till in the pull-down that occurs on breaking through the bottom of the ice and into the basal water system, as described later. Ice deformation, till deformation, and basal sliding. The measurement of internal deformation of the ice, even the lowermost part, cannot be carried out accurately enough in one short field season. For this reason, one borehole was left filled with antifreeze in the hope that it can be reentered and the ice deformation can be measured in the next field season. A tubular mechanical tiltmeter was drilled 0.5 meter into the till and left there 4 hours; upon removal the tube was bent 2 ANTARCTIC JOURNAL
Ice flow on deforming sediments: Ice stream Band Lake Michigan? RICHARD B. ALLEY
Earth System Science Center
and
Department of Geosciences Pennsylvania State University University Park, Pennsylvania 76802
The existence of a deforming bed beneath ice stream B (Alley et al. 1986) remains probable but unproven. Identification of the basal mechanism of ice-stream flow will allow us to concentrate on finding its parameters for numerical models, a development which will lead to improved predictions of the rate at which the ice sheet will respond to perturbations such as those possibly caused by greenhouse warming or those already triggered by past climatic changes. Ongoing field studies associated with the Siple Coast Project seek to identify the mechanism of ice-stream lubrication unequivocally (Engelhardt et al. 1990). Over the last year, I refined a possible one-dimensional steady model for basal behavior and found that this model is consistent with available data (Alley 1990); this consistency continues to favor the deforming-bed hypothesis. This model suggests that ice streaming is initiated when enough basal meltwater accumulates to raise basal water pressures quite close to flotation; a poorly consolidated bed that erodes easily to produce 1990 REVIEW
This research was supported by National Science Foundation grant DPP 87-16604.
References
Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1986. Seismic measurements reveal a saturated porous layer beneath an active Antarctic Ice Stream. Nature, 322, 54-57. Engelhardt, H.N. Humphrey, B. Kamb, and M. Fahnstock. 1990. Physical conditions at the base of a fast moving Antarctic Ice Stream. Science, 248, 57-59.
Harrison, W.D., and K.A. Echelmeyer. 1989. Short-term variations in the speed of ice stream B, Antarctica. Antarctic Journal of the U.S., 24(5), 81-82.
McDonald J., and I Whillans. In press. Search for short-term velocity changes of ice stream B, West Antarctica. Journal of Glaciology. Sanderson, T.J.O. 1978. Thermal stresses near the surface of a glacier. Journal of Glaciology, 20, 257-283.
sediments enhances lubrication and causes ice streaming upglacier of the point where it would begin over hard bedrock. If the geothermal flux is low or subglacial aquifers are efficient, ice streaming can be suppressed even if the bed is melted. There may be two steady states for ice-surface profiles in some regions: • a steep surface slope that causes rapid internal ice deformation but drives basal meltwater through subglacial aquifers so it does not lubricate ice flow, and • a gentle surface slope that slows internal ice deformation but allows meltwater to accumulate at the bed and lubricate basal sliding or bed deformation. The next step in this modeling effort, now in progress, is to conduct time-dependent simulations and check for consistency with available data. If the model still appears to be adequate, then we can begin to assess the stability of the ice-stream system. Of course, further data from the ongoing Siple Coast field programs are the most direct and valuable test of icestream dynamic controls, but modeling provides a useful adjunct. The probability of a deforming bed beneath a large antarctic ice stream raises further interest in the fast-moving ice that drained the Laurentide ice sheet during the Late Wisconsinan, and that deposited widespread till sheets. The antarctic deforming-bed model leads to a better understanding of how the deposits of a deforming-bed glacier might.appear. Comparison of predictions of the deforming-bed model and of other basal models with observed characteristics of basal tills deposited by the Lake Michigan lobe in Illinois shows that the deformingbed model fits the observations better than other models. It is an interesting possibility that ice stream B and the Lake Michigan lobe may be quite similar in ice and sediment dynamics (Alley et al. 1988; Alley in press). This work was supported in part by National Science Foundation grant DPP 87-16016. 79
References Alley, R.B., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1986. Deformation of till beneath ice stream B, Antarctica. Nature, 322, 5759. Alley, R.B., D.D. Blankenship, S.T. Rooney, and C.R. Bentley. 1988. Glacial deposits as (lack of) evidence for deforming beds. (Abstract). SEPM Midyear Meeting Abstracts, 5, 2.
Borehole geophysical observations on ice stream B, Antarctica HERMANN ENGELHARDT, NEIL HUMPHREY, and BARCLAY KAMB Division of Geological and Planetary Sciences California Institute of Technology Pasadena, California 91125
In recent years, the development of the hot-water drilling technique for rapidly drilling deep boreholes through cold ice to the base of the antarctic ice streams has opened the possibility to study the controlling mechanism for fast ice streaming flow. This technique has been successfully tested and implemented during the 1988-1989 austral summer (Engelhardt et al. 1989, 1990; Kamb 1990). In the 1989-1990 austral summer, six boreholes about 1,060 meters deep were drilled on ice stream B (83.5°S 138.2°W). In this article, we report on the geophysical experiments and observations made in these boreholes. Sampling of subglacial material. Samples for studying the physical properties of bottom materials were extracted by four different methods. Two of them, the jet sampling technique and the adhesion method, were already used in the previous field season (Engelhardt et al. 1989). This year, two new devices were added: • The split-tube corer—a short, split piece of heavy tubing, 62 millimeters in diameter—is driven into the bottom sediments using a heavy weight. The split tube can be readily opened for removal of the core material. Samples of subglacial sediment 0.2 meter long were obtained. • The piston corer is 6.5 meters in overall length. The core tube, 50 millimeters in diameter with piston to fit, is 3.5 meters long; the rest of the length is a piece of heavy steel (Shelby) tubing to drive the corer into the bottom. Although the piston corer is more difficult to handle than the other sampling devices, this sampling technique proved to be highly advantageous. Four cores were retrieved: lengths 1.3, 2.0, 2.0, and 3.4 meters. Preliminary examination of the cores show the general character of the material. It is a glacial till—a pebbly, sandy, silty clay with a wide range of grain sizes typical of till, and lacking bedded structure. Clasts up to 5 centimeters in size are present; they are mainly granitic, but metamorphic lithologies are also present. Scarce shell fragments are visible macroscopically, 80
Alley, R.B. 1990. Multiple steady states in ice-water-till systems. Annals of Glaciology, 14, 1-5. Alley, R.B. In press. Deforming-bed origin for southern Laurentide till sheets? Journal of Glaciology. Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving Antarctic ice stream. Science, 248, 57-59.
and microscopic organic remains (sponge spicules, diatoms, etc.) are present though not abundant (Scherer 1989). The till, therefore, is derived, at least in part, from open water marine sediments. A wide variety of source materials has been mixed together, as can be expected in a till. The directly measured porosity of a sample from near the top of the till is 40 percent. This value is high for a till and probably indicates that the till has been dilated by recent shear. A hydraulic conductivity of 2 x iO per millisecond was measured on a reconstituted sample. This low value, typical of tills, precludes hydraulic conduction through the till layer as a significant contribution to water flow at the base of the ice stream. Preliminary mechanical tests have been carried out on till samples from which the coarser rock fragments (greater than 5 millimeters) have been removed. The shear strength is 0.020.04 bar, varying from sample to sample. The till behaves like a perfectly plastic material with this low failure strength. Because of the low hydraulic conductivity, the shear strength measured on the core samples is probably close to the in situ value under the ice stream. The strength of the till is so low that the till should be deforming under the ice, if the basal shear stress approximates the regional average value of 0.2 bar. In fact, the till is by a wide margin too weak to support the average basal shear stress. Therefore, the mechanical support and stability of the ice stream are called into question at the locality studied (Upstream B). The areal coverage of the investigation must be widened to provide a set of representative basal conditions broad enough to account for overall mechanical equilibrium of the ice stream. Till thickness. The length of the longest core extracted from the till shows that the till is at least 3.4 meters thick. The depth of the till was sounded by using the hot-water drill, which penetrates into the till by hydraulic action. The drill went 5 meters into the till without any indication of encountering a consolidated, impenetrable bottom. In two boreholes the drill stem penetrated at least its full length (3.6 meters) into the till in the pull-down that occurs on breaking through the bottom of the ice and into the basal water system, as described later. Ice deformation, till deformation, and basal sliding. The measurement of internal deformation of the ice, even the lowermost part, cannot be carried out accurately enough in one short field season. For this reason, one borehole was left filled with antifreeze in the hope that it can be reentered and the ice deformation can be measured in the next field season. A tubular mechanical tiltmeter was drilled 0.5 meter into the till and left there 4 hours; upon removal the tube was bent 2 ANTARCTIC JOURNAL
